Several hydraulic drive systems have been proposed for a work machine such as a hydraulic excavator. These hydraulic drive systems each include a plurality of main pumps, and perform load sensing control of at least one of the plurality of main pumps to achieve both excellent combined operability and energy saving.
For example, Patent Document 1 proposes a following structure.
A hydraulic drive system of a work machine such as a hydraulic excavator includes first and second pumps constituted by two delivery ports of a split flow type pump of a variable displacement type, and a third pump of a fixed displacement type. During non-traveling operation, the hydraulic drive system combines hydraulic fluids of the first and second pumps, and supplies the fluids to a front implement actuator to perform load sensing control. During swing operation, the hydraulic drive system supplies a hydraulic fluid of the third pump of the fixed displacement type to a swing motor via an open center circuit. In case of an operation for traveling only, or a simultaneous operation for operating actuators other than the front implement, such as operation for traveling and swing, the hydraulic fluids of the first and second pumps are supplied to left and right traveling motors via the open center circuit, while the hydraulic fluid of the third pump is supplied to the swing motor via the open center circuit. In case of a combined operation for traveling and the front implement, the hydraulic fluids of the first and second pumps are supplied to the left and right traveling motors, while the hydraulic fluid of the third pump is supplied to the front implement actuators. The hydraulic fluids in the combined operation are supplied via corresponding pressure compensating valves and flow control valves to perform split flow control using the pressure compensating valves.
Patent Document 2 proposes a following structure.
A hydraulic drive system of a work machine such as a hydraulic excavator includes first and second pumps constituted by two delivery ports of a split flow type pump of a variable displacement type, and a third pump of a variable displacement type. Each of the first and second pumps and the third pump is configured to perform load sensing control. Torque of the third pump is detected by approximation using two pressure reducing valves, and fed back to the first and second pumps. A hydraulic fluid of the third pump is used for main driving of a boom cylinder, while a hydraulic fluid of the first pump is used for assist driving of the boom cylinder. A hydraulic fluid of the second pump is used for main driving of an arm cylinder, while a hydraulic fluid of the first pump is used for assist driving of the arm cylinder.